<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

        .main {
            background-color: black;
        }
        .a,.b,.c{
            float: left;
            height: 100px;
            width: 33%;
        }
        .a {

            background-color: #3c763d;
        }
        .b {
            background-color: blue;
        }
        .c {
            background-color: red;
        }

    </style>
</head>
<body>
<div class="main">
    <div class="a"></div>
    <div class="b"></div>
    <div class="c"></div>
</div>
<h2 style="background-color: yellow">这是一个网页</h2>
</body>
</html>